In general, the preparation of a magnetic toner is effected in the following manner. In some detail, a magnetic substance, a binder resin, a charge controlling agent, etc. are homogeneously mixed by means of a mixer such as ball mill and Henschel mixer, and then melt-kneaded by means of a kneading machine such as roll mill, extruder and kneader to cause the various constituent components to be fully dispersed in the binder resin. After a proper period of time, the material is finely divided by means of a jet mill or the like, and then classified by means of a pneumatic classifier to obtain a magnetic toner.
In order to enhance the dispersibility of a magnetic substance in a binder resin, an approach has been proposed which comprises the treatment of the surface of the magnetic substance with various substances for the purpose of enhancing the compatibility of the magnetic substance in an organic substance. For example, JP-A-53-81125 (The term "JP-A" as used herein means an "unexamined published Japanese Patent application") discloses a high molecular material as such a treatment. JP-A-54-127329 discloses a silane coupling agent. JP-A-55-28019 discloses a titanium coupling agent. These treatments are excellent in the enhancement of compatibility.
In a magnetic toner, however, a magnetic substance is adapted to be exposed somewhat on the surface of the toner so that excess electric charge is released to attain a proper electric charge. Therefore, the use of the foregoing treated magnetic substance may impair the surface hydrophilicity of the magnetic substance as well as give a greater polarization that prevents the release of electric charge. This will cause the toner to be excessively charged and scattered on the image, resulting in image roughness. This may also cause a density drop or unevenness in sleeve coat. This phenomenon is unavoidable under low humidity or in a high speed developing machine.
On the other hand, various magnetic substances have been prepared to obtain desired magnetic properties, electrical properties and powder properties. However, some of these magnetic substances cannot sufficiently release electric charge. Many magnetic substances cause image defects, though its extent is small as compared with the foregoing magnetic substances.
As approaches for enhancing the dispersibility of a magnetic substance there have been disclosed surface active agents in JP-A-53-137148, JP-A-59-126544, JP-A-59-125748, JP-A-59-137595, and JP-A-61-59349. These approaches are excellent in the enhancement of compatibility with a binder resin and thus provide stable electrification that gives a good image even under low humidity. Under high humidity, however, these approaches are often liable to the increase in the release of electric charge that causes a density drop, though being moisture-resistant in that no irreversible changes occur.
Further, a polymerization process has recently been employed to prepare a toner. Even the polymerization process requires the enhancement of pigment dispersibility. Examples of known polymerization processes include a process which comprises the use of a titanium or silane coupling agent as disclosed in JP-A-58-293759, JP-A-64-44456, and JP-A-2-32364, a process which comprises the use of an ester as disclosed in JP-A-59-125747, and JP-A-2-269365, and a process which comprises the use of physical dispersion as a means as disclosed in JP-A-2-256061. However, all these approaches, though providing a significant improvement in the dispersibility of a magnetic substance itself in a polymerizable monomer, are not necessarily satisfactory in that they have adverse effects on the formation of oil drops during suspension, cause heating during dispersion that leads to the deterioration of the polymerizable monomer, or have adverse effects on the charging behavior or electrical properties of the finished toner.
As previously mentioned, various proposals have been made on grinding process toner having an improved pigment dispersibility. However, no satisfactory polymerization processes have been obtained.